This invention was made with Government support. The Government has certain rights in this invention.
1. Field of the Invention
The present invention relates to systems and methods for spacecraft navigation, and in particular to a spacecraft attitude determination system for correcting gyro scale factor non-linearity.
2. Description of the Related Art
Satellite navigation systems typically include an attitude determination system. Attitude determination systems typically comprises one or more rate sensors (which provide measurements of the rotation rate of the spacecraft) and one or more attitude sensors (which measure the attitude of the spacecraft). Rate sensors may include mechanical gyros, ring laser gyros (RLGs), or similar devices. Attitude sensors include, for example, sun sensors, earth sensors, and/or star trackers. Typically, sun and earth sensors are used to provide a rough spacecraft attitude estimate (useful, for example in recovering from large spacecraft motions), while the star trackers provide a more accurate attitude estimates.
For agile spacecraft, gyro scale factor and misalignment errors contribute significantly to spacecraft attitude determination errors. To ameliorate this problem, a Kalman filter can be used to generate estimates of the spacecraft attitude, gyro scale factors and gyro misalignments. Using such estimates, substantial performance improvements can be obtained. However, the Kalman filters used in such designs typically assume that the gyro scale factors are constant across different measured angular rates. What is needed is an attitude control system that compensates for rate sensor scale factor errors, without rendering the Kalman filter algorithms unnecessarily complex. The present invention satisfies that need.
To address the requirements described above, the present invention discloses a method and apparatus for calibrating rate sensor measurements to compensate for rate sensor scale factor non-linearities. The method comprises the steps of generating a current rate sensor scale factor estimate; generating a deviation of the scale factor estimate from a current scale factor non-linearity correction mapping; generating an updated scale factor non-linearity correction mapping from the deviation of the scale factor estimate from the current scale factor non-linearity correction mapping; and mapping the rate sensor measurement according to the current scale factor non-linearity correction mapping. The apparatus comprises a plurality of modules that can be implemented in hardware or in software by one or more processors. The modules include a first module for estimating a gyro scale factor; a second module, communicatively coupled to the first module, for generating a deviation of the scale factor estimate from a current scale factor non-linearity correction mapping; a third module, communicatively coupled to the second module for generating an updated scale factor non-linearity correction mapping from the deviation of the scale factor estimate from the current scale factor non-linearity correction mapping; and a fourth module, communicatively coupled to the third module, for mapping the rate sensor measurement according to the current scale factor non-linearity correction mapping.